With the advent of silicon michromachining processes for forming silicon micromachined sensors, improved sensor performance, reductions in cost and improved sensitivity to non-electrical input parameters for conversion into electrical signals have been achieved. Generally, silicon micromachining enables precision formation of three-dimensional shapes in silicon and includes various process steps employing photolithography, etching and thin-film deposition. Silicon micromachined sensors have been used and have the capability for use in many environments. For example, electronic fuel management systems in automobiles have to be able to rely on sensors, for example, in the fuel air stream, which are rugged and reliable and which have fast response times. More generally, fluid flow sensors employing silicon micromachining techniques have been developed to measure parameters such as pressure, acceleration, vibration, radiation and the like. Where these sensors have been employed in fluid flow environments, however, the sensors are normally mounted on a substrate in overlying relation where they project from the substrate surface. Additionally, the electrical connections between the fluid flow sensitive electrical signal producing elements of the sensor and the electrical interconnects for the signal processing means stand out or project from the substrate. These types of sensor and electrical connection mountings interfere with the flow of the fluid over the substrate. Consequently, the measured fluid flow parameters may not accurately reflect the actual fluid flow and may require correction for such interference. For example, where a mass airflow sensor is mounted on an airfoil in a fluid flow stream in a confined tube or chamber, the projecting sensor would create a turbulence about the area of the sensor which is required to measure the static pressure of the flow past the sensor. Consequently, the measured static pressure would be different than the actual static pressure of the fluid in the same region in the absence of a projecting sensor. Also, by projecting from the substrate surface, particles within the flow may impact and adhere to the sensor and its electrical interconnections to the extent of interfering with its sensing function or at least inhibiting the sensitivity of the sensor.
Prior mass fluid-flow sensors employing silicon micromachining techniques have been mounted to project from the substrate with an overlying hood structure used to channel the fluid over the sensing elements of the sensor. The hood structure has, to some extent, eliminated the problems associated with the non-covered projecting sensor but has not eliminated them in their entirety. As a result, there has developed a need for silicon micromachined fluid flow sensors which do not interfere with, or minimize their interference with, the fluid flow passing over the sensing area and which inhibits or directly eliminates the impact and adherence of particles in the fluid flow on the sensor per se such that its sensitivity to the parameters being measured remains substantially constant.